You will see this warning if you have no caches installed and enabled, or if
your cache does not have the required minimum version installed; older versions
are disabled because of performance problems.

If you see “{Cache} below version {Version} is installed. for stability and
performance reasons we recommend to update to a newer {Cache} version” then
you need to upgrade, or, if you’re not using it, remove it.

You are not required to use any caches, but caches improve server performance.
See Memory Caching.

“You are accessing this site via HTTP. We strongly suggest you configure your
server to require using HTTPS instead.” Please take this warning seriously;
using HTTPS is a fundamental security measure. You must configure your Web
server to support it, and then there are some settings in the Security
section of your ownCloud Admin page to enable. The following pages
describe how to enable HTTPS on the Apache and Nginx Web servers.

“cURL is using an outdated OpenSSL version (OpenSSL/$version). Please update your
operating system or features such as installing and updating apps via the
ownCloud Marketplace or Federated Cloud Sharing will not work reliably.”

“cURL is using an outdated NSS version (NSS/$version). Please update your operating
system or features such as installing and updating apps via the ownCloud
Marketplace or Federated Cloud Sharing will not work reliably.”

There are known bugs in older OpenSSL and NSS versions leading to misbehaviour in
combination with remote hosts using SNI. A technology used by most of the HTTPS
websites. To ensure that ownCloud will work properly you need to update OpenSSL
to at least 1.0.2b or 1.0.1d. For NSS the patch version depends on your distribution
and an heuristic is running the test which actually reproduces the bug. There
are distributions such as RHEL/CentOS which have this backport still pending.

Your Web server is not set up properly to resolve /.well-known/caldav/ or /.well-known/carddav/¶

Both URLs need to be correctly redirected to the DAV endpoint of ownCloud. Please
refer to Service discovery for more info.